Malathion ([(dimethoxyphosphinothioyl)thio]butanedioic acid diethyl ester; CAS # 121-75-5) is an organophosphate insecticide that inhibits cholinesterase activity in vivo. Malathion has the following chemical structure:

Malathion may be prepared by reacting O,O-dimethyldithiophosphoric acid (DMDP) with diethyl maleate (U.S. Pat. Nos. 2,578,652, 2,879,284, 3,403,201, 3,463,841, 3,470,272, 4,367,180 and 4,681,964).

Numerous impurities are found in malathion preparations; these impurities include, O,O,S-trimethyl phosphorodithioate (MeOOSPS), O,O,S-trimethyl phosphorothioate (MeOOSPO), O,S,S-trimethyl phosphorodithioate (MeOSSPO), malaoxon, isomalathion, diethyl fumarate, methyl malathion, dimethyl malathion, O,O-methyl,ethyl-S-(1,2-dicarboethoxy)ethyl-phosphorodithioate, and tetraethyl dithiodisuccinate (See, WHO Specifications and Evaluations for Public Health Pesticides: Malathion, World Health Organization, 2003). Some of these impurities are formed as breakdown products during storage, but, the majority of these impurities are generated as unintentional byproducts during synthesis. (Health Risk Assessment of Malathion Coproducts in Malathion-Bait Used for Agricultural Pest Eradication in Urban Areas, Report of the California Environmental Protection Agency, 1997). For example, during storage, malathion can convert to isomalathion by dimerization, and the extent of isomerization is dependent on particular storage conditions. (Health Risk Assessment of Malathion Coproducts in Malathion-Bait Used for Agricultural Pest Eradication in Urban Areas, Report of the California Environmental Protection Agency, 1997).
Storage of malathion at elevated temperatures, e.g., 40° C., significantly enhances toxicity of the malathion preparation (Umetsu et al., J. Agric. Food Chem., 25:946-953 (1977)). In part, this enhancement is due to an increase in isomalathion after storage. For example, after storage for 6 months at 40° C., there was an increase in isomalathion content from 0.2% to 0.45%, with an accompanying 35% increase in toxicity as measured by LD50 in mice (Umetsu et al., J. Agric. Food Chem., 25:946-953 (1977)). Because even small or trace quantities of malathion impurities such as isomalathion have been shown to be highly toxic, the presence of these impurities in any malathion preparation, but especially one developed for pharmaceutical use, should be reduced as much as possible. Moreover, given that malathion breaks down into toxic by-products during storage, it is also desirable to prepare malathion which is storage stable.
Many of these malathion impurities have been found to be toxic. MeOOSPO and MeOSSPO can cause liver damage (Keadtisuke et al., Toxicology Letters 52:35-46 (1990)), or immune suppression (Rodgers et al., Immunopharmacology 17:131-140 (1989)). Isomalathion has been shown to cause death in people after spraying during insect eradication programs. (Aldridge et al., Archives Toxicology 42:95-106 (1979)). The toxicity of isomalathion is due to its ability to inhibit acetylcholinesterase; in fact, isomalathion is approximately 1,000 times as active against acetylcholinesterase as compared with malathion. (Berkman et al. Synthesis of Chiral Malathion and Malathion, Terahedron Letters 33(11):1415-1418 (1992)). O,O-methyl,ethyl-S-(1,2-dicarboethoxy)ethyl-phosphorodithioate, isomalathion and MeOOSPO all exhibit pulmonary toxicity and can cause death from hypoxia. (Imamura et al., Pharmacology and Therapeutics 38(3):419-427 (1988)). Malaoxon inhibits cholinesterase enzymes. (Umetsu et al., J. Agric. Food Chem., 25:946-953 (1977)). Diethyl fumarate can cause contact urticaria. (Maibach, Contact Dermatitis 12(3):139-140 (1985)). Malathion's physical properties make it difficult to remove impurities by conventional means. For example, because malathion is a liquid at ambient temperature (melting point=2.9° C.), crystallization is difficult. Malathion also has a high boiling point (156-157° C.), consequently, distillation also has its problems, especially as malathion is unstable at elevated temperatures.
We have now developed a novel method for synthesizing and purifying malathion for pharmaceutical use. The malathion prepared by the methods of this invention has significantly lower levels of toxic impurities such as isomalathion when compared with other, commercially available malathion preparations that are currently used for pharmaceutical purposes. Moreover, because malathion is known to be unstable, the levels of toxic impurities, e.g., isomalathion, are known to increase over time, there is a need to develop a stable form of malathion. The malathion of the present invention is storage stable, i.e., even after storage at elevated temperature and humidity, the levels of toxic impurities do not increase significantly.